16 September 2013

Is climate change already dangerous? (1)

… the (climate) disruption and its impacts are now growing much more rapidly than almost anybody expected even a few years ago. The result of that, in my view, is that the world is already experiencing ‘dangerous anthropogenic interference in the climate system’… The question now is whether we can avoid catastrophic human interference in the climate system.
— John Holdren, senior advisor to President Barack Obama on science and technology issues, 2008

The stated purpose of international climate negotiations is to avoid “dangerous” climate change or, more formally, to prevent “dangerous anthropogenic interference with the climate system”. Most of the climate action movement and most NGOs identify with this goal.
But if climate change is already “dangerous”, what then is our purpose?

To return the planet to a safe climate (Holocene conditions)?

To accept that climate change is already irretrievably dangerous state of affairs? In which case the purpose instead becomes …

To prevent a plunge into an even worse “catastrophic” breakdown of human society and planetary and climate system elements?

And if conditions existing today for some elements of the climate system and the existing greenhouse gas levels and radiative forcing are already sufficient to push more climate system elements past their tipping points and create “catastrophic” breakdown without any further emissions, what then is our purpose?

This paper sets out the evidence that dangerous climate change has already occurred and canvasses possible responses.

1. “DANGEROUS” METRICS

1a. Safe boundary

A landmark research paper by Rockstrom, Steffen et al. in 2009 established that “human activities have reached a level that could damage the systems that keep Earth in the desirable Holocene state… The result could be irreversible and, in some cases, abrupt environmental change, leading to a state less conducive to human development…” They observed that “a new era has arisen, the Anthropocene, in which human actions have become the main driver of global environmental change”.

To meet the challenge of maintaining the Holocene state, the authors proposed a framework based on “planetary boundaries” which “define the safe operating space for humanity with respect to the Earth system and are associated with the planet’s biophysical subsystems or processes”. The boundaries are “values for control variables that are either at a ‘safe’ distance from thresholds — for processes with evidence of threshold behaviour — or at dangerous levels — for processes without evidence of thresholds”.

The authors proposed nine boundaries, including a climate boundary that “human changes to atmospheric carbon dioxide (CO2) concentrations should not exceed 350 parts per million (ppm) by volume, and that radiative forcing should not exceed 1 watt per square metre (W/m2) above pre-industrial levels”.

But CO2 concentrations now exceed 400 ppm by volume, and the 2007 IPCC report estimated greenhouse gas forcings of 3 (2.5–3.5) W/m2 above pre-industrial levels. By this metric, climate change is now clearly dangerous, exceeding the safe boundary by wide margins: more than 50 ppm CO2 (equivalent to +0.5ºC of warming) and by more than 1-2 W/m2.

1b. “Burning embers”: five concerns

Figure 1: The updated “reasons for concern”

The “burning embers” diagram of the third IPCC report in 2001 was revised and updated by Smith, Schneider et al. in 2009, and will be updated again in the new 2014 IPCC report to include the colour purple to indicate worsening climate risks. It provides five “reasons for concern”:

A tipping point is a step change, or passing of a critical threshold, in a major earth-climate system component, where a small push or change unleashes a bigger change in the component through positive feedbacks, which amplify the change. The classic case in global warming is the ice–albedo feedback, where decreases in the ice cover area change surface reflectivity, trapping more heat and producing a temperature rise and further ice loss. A discussion of tipping points and the limitations of current tipping point science may be found a future post in this series.

This overview focuses on Arctic tipping points (concern 1. above). It is beyond this paper’s scope to provide comprehensive and robust evidence for all five concerns, but one can note in passing that recent climate-change impacted extreme weather events, such as Superstorm Sandy, would reasonably fall within the definitions of concerns 2. and 4. The disproportionate and sizeable impacts of climate change on poor and developing nations, which have already been documented by UN agencies and aid organisations, constitute reasonable evidence for concern 3. The imminent loss of most of the world’s coral reef systems clearly qualifies under 1., and so on.